Human bronchus, colon, duodenum, esophagus, and pancreatic duct cultured either as explants of epithelial cells in chemically defined media provide an excellent in vitro system to study the metabolism of chemical carcinogens, including those found in tobacco smoke and the environment. Several classes of chemical carcinogens, polynuclear aromatic hydrocarbons, N-nitrosamines, hydrazines, aromatic amines, and mycotoxins, can be metabolically activated by human tissues. Fetal human liver, stomach, and esophagus cultured as explants metabolized the same group of compounds. The metabolic pathways leading to the formtion of DNA adducts in explants and epithelial cell cultures have been defined for benzo[a]-pyrene (BP), 7,12-diamethylbenz[a]anthracene, aflatoxin B1 (AFB), and N,N- dimethylnitrosamine (DMN). The adducts between these carcinogens and DNA in human tissues are essentially the same as those found in experimental animals in which the chemicals are carcinogenic. Interindividual differences in carcinogen-DNA binding values vary 50- to 150-fold. Studies of the metabolism and DNA adduct formation of 1-nitropyrene and 6-nitrobenzo[a]pyrene has also shown similar findings in interindividual and interspecies comparisons. The role of AFB in liver carcinogenesis has been further studied. Fetal liver explants have also been shown to activate AFB, DMN and BP. Since human fetus may be transplacentially exposed to these carcinogens, this finding may be important in the etiology of liver cancer and the early age of onset in China. We found that when urine samples collected in Kenya were analyzed for the presence of 2,3-dihydro-2-(7'-guanyl)-3-hydroaflatoxin B1 ((AFB-Gua I) by high pressure liquid chromatography, 11 of 128 samples had a detectable level of AFB-Gua I; its identity was confirmed primarily from people living in low-lying areas and colelcted in the rainy season when AFB contamination of the food is highest.